FIG. 5 is a schematic view showing the construction of a conventional disconnection detection apparatus. FIG. 6 is a circuit diagram showing main portions of the disconnection detection apparatus shown in FIG. 5.
As shown in FIGS. 5 and 6, a disconnection detection apparatus 100 is interposed between a switch SW and a load F of a feeding path 2. The feeding path has a power supply 4, a fuse H, the switch SW, and the load F connected in series with one another. Let it be supposed that the load F is a lamp.
The disconnection detection apparatus 100 has a pair of terminals P1 and P2 connected to a wire such as a wire harness extending from the power supply 4 and a wire such as a wire harness extending from the load F, respectively. A detection resistor R1 is interposed and connected between the terminals P1 and P2. A detection part (first detection part) detects a disconnection of the feeding path 2, according to a decrease of a voltage drop at the detection resistor R1 compared to a voltage drop which may occur in normal operation. A determination part 3 determines whether the feeding path 2 has a disconnection, based on a detection result supplied by the detection part A. A warning lamp 5 informs an operator of the disconnection of the feeding path 2. A driving part 7 drives the warning lamp 5. A control part 9 controls the driving part 7, based on a determination result supplied by the determination part 3.
As shown in FIG. 6, the detection part A includes resistors R2 and R3, a transistor (for example, PNP type) Q1 and Q2, and a capacitor C1. In the detection part A, an electric wire is branched at an upstream end of the detection resistor R1 and is connected in series with the resistor R2, the emitter-collector of the transistor Q1, and a resistor R3. A wire extending from the resistor R3 is grounded through a diode (forward connection) D1. An electric wire is branched at a downstream end of the detection resistor R1 and is connected with the emitter-collector of the transistor Q2. The emitter-collector of the transistor Q2 is connected with the anode of the diode D1 through the determination part 3 which will be described later. A base of the transistor Q1 is connected to the collector and a base of the transistor Q2. The noise-removing capacitor C1 is connectedly interposed between the downstream end of the detection resistor R1 and a downstream end of the resistor R2.
The detection part A and the detection resistor R1 form a current Miller circuit. As will be described later, the transistor Q2 is turned on and off according to a change of an electric potential (namely, electric potential of emitter of transistor Q2) at the downstream end of the detection resistor R1. In a disconnection detection state, that is generated when the transistor Q2 is on, electric current supplied by the power supply 4 is fed to the determination part 3 through the transistor Q2 as a detection result indicating that a disconnection has occurred.
As shown in FIG. 6, in the determination part 3, a base of a transistor (for example, NPN type) Q3 is connected to the collector of the transistor Q2 through a resistor R4. Further an emitter of the transistor Q3 is branch-connected to the base of the transistor Q3 through a resistor 5 and to the anode of the diode D1. Further the collector of the transistor Q3 is connected to the terminal P1 through voltage-dividing resistors R6 and R7. In addition, a noise-removing capacitor C2 is connected in parallel with the resistor R5. Further in the determination part 3, a base of a transistor (for example, PNP type) Q4 is connected to a node between resistors R6 and R7. An emitter of the transistor Q4 is connected to the terminal P1. A collector of the transistor Q4 is connected to the control part 9.
As will be described later, the transistor Q3 is turned on and off by the electric current fed from the transistor Q2 of the detection part A as a detection result which indicates that a disconnection has occurred. In a disconnection-determined state, which is generated when the transistor Q3 is on, the electric current supplied by the power supply 4 is fed to the control part 9 through the transistor Q4 as a determination result which indicates that the feeding path 2 has been determined to have a disconnection.
Upon receipt of the electric current, which indicates that the feeding path 2 has been determined as having a disconnection from the determination part 3, the control part 9 turns on the warning lamp 5 through the driving part 7.
The operation of the disconnection detection apparatus 100 is described below. Let it be supposed that the switch SW is ON. In a normal operation in which a load current supplied by the power supply 4 is grounded via the fuse H, the switch SW, the detection resistor R1, and the load F, the electric potential of the emitter of the transistor Q2 drops below a threshold because a voltage drop at the detection resistor R1 is caused by the flow of the load current therethrough. Thus the voltage between the emitter and base of the transistor Q2 drops below an ON-voltage, and the transistor Q2 is turned off. In this case, the transistors Q3 and Q4 are kept in an off state, and the electric current which indicates that the feeding path 2 has been determined to have a disconnection is not fed from the determination part 3 to the control part 9. Therefore the control part 9 does not operate and the warning lamp 5 does not turn on.
When the feeding path 2 has a disconnection (normal disconnection of feeding path 2, for example, disconnection of a wire at downstream side of terminal P2), the load current does not flow through the detection resistor R1. Thus no voltage drop occurs in the detection resistor R1. That is, the voltage of detection resistor R1 drops below the voltage drop threat in normal operation. In this case, the electric potential of the emitter of the transistor Q2 is increased to a supply potential. Further the voltage between the emitter and base of the transistor Q2 rises over the ON-voltage, and the transistor Q2 is turned on (disconnection is detected). Thereby the electric current supplied by the power supply 4 flows to the ground as a result of detection of disconnection through the detection resistor R1, the transistor Q2, the resistors R4 and R5, and the diode D1. Due to a voltage drop at the resistor R5, which is caused by the flow of the electric current therethrough, the voltage between the emitter and base of the transistor Q3 rises. As a result, the transistor Q3 is turned on, determining that disconnection has occurred, and the resistors R7 and R6 pass electric current therethrough. Due to the voltage-dividing resistance of the resistors R7 and R6, the voltage between the base and the emitter of the transistor Q4 rises. As a result, the transistor Q4 is turned on, and the electric current supplied by the power supply 4 is fed to the control part 9 through the transistor Q4. Upon receipt of the electric current, the control part 9 drives the warning lamp 5 through the driving part 7.
To detect the disconnection accurately and prevent a malfunction from occurring, in the disconnection detection apparatus 100, it is necessary to increase the value of the detection resistor R1 and the voltage drop at the detection resistor R1. To make the disconnection detection apparatus 100 compact and reduce the cost, it is also necessary to reduce an allowable loss of the detection resistor R1.
However, in the case where the above-described two conditions are satisfied, an open destruction, electrically unconductive at the detection resistor R1, may occur. That is, if the feeding path 2 is short-circuited with a peripheral member at a point G (shown in FIG. 6), a short-circuit current supplied by the power supply 4 and grounded through the fuse H, the switch SW, the detection resistor R1, and the short-circuit point G does not flow through the load F. Therefore the load current short-circuited is higher than the load current not short-circuited. In this case, when the resistance value of the detection resistor R1 is set to a large value for the above-described reason, the short-circuited current is suppressed and will not become high. In the case where the short-circuited current flows through the feeding path 2, it takes a long time for fusion of the fuse H. Thus the short-circuited current flows through the detection resistor R1 for a long time. Further since the resistance value of the detection resistor R1 is set to a large value, a large amount of heat is generated at the detection resistor R1. On the other hand, in the case where the allowable loss of the detection resistor R1 is set to a small value for the above-described reason, the amount of the generated heat exceeds the allowable loss and the open destruction occurs at the detection resistor R1.
However, in the state where the detection resistor R1 has the open destruction, the downstream end of the detection resistor R1 has the ground potential, and the emitter potential of the transistor Q2 is kept below the threshold, and the transistor Q2 is kept in an off state. Thus the detection part A does not detect the open destruction. Accordingly the disconnection detection apparatus 100 has a disadvantage that the open destruction which has occurred at the detection resistor R1 cannot be detected.